1. Field of the Invention
This invention relates to a semiconductor device and a manufacturing method thereof and more particularly to a laser dicing technique for applying a laser beam to divide a semiconductor wafer into discrete chips.
2. Description of the Related Art
It is predicted that a microprocessor now used is required to process a further larger amount of information at high speed in future. So far, miniaturization of transistors determines the performance of the microprocessor. However, in recent years, RC delay (R: Resistance, C: Capacitance) causes a problem and much importance is given not only to miniaturization of transistors but also to parasitic capacitance (capacitance between wirings arranged with an insulating material disposed therebetween) and resistance of wirings which connect transistors to one another.
In order to suppress the RC delay, it becomes necessary to change a wiring material from Al to Cu and use a material with a small dielectric constant instead of a silicon oxide film as an insulating material. However, the insulating film with the small dielectric constant generally has a porous structure and since the low dielectric property is acquired by virtue of the structure, the mechanical strength and adhesion strength thereof are extremely low in comparison with those of the silicon oxide film. Therefore, when a semiconductor wafer is diced into discrete chips, layer-layer separation tends to occur if the insulating film with the small dielectric constant is mechanically cut. Further, since the Cu wiring (or copper conductor) is formed of a material having relatively high viscosity, film separation tends to occur if a normal blade dicing process is performed.
Therefore, much attention is paid to laser dicing instead of the conventional blade dicing. In the laser dicing, a laser beam with high energy is applied to melt and cut a semiconductor wafer. Therefore, it is expected that the cutting property of the Cu wiring and insulating film with the small dielectric constant can be significantly enhanced in comparison with the grinding method such as the conventional blade dicing.
As the laser dicing, the following two methods are considered. The laser dicing methods are described in Jpn. Pat. Appln. KOKAI Publication No. 2002-192367, for example. The first method is to apply a laser beam after focusing the laser beam on the uppermost layer by use of a lens (condenser lens) 11 as shown in FIG. 1 and melt and cut a wafer 12 together with a circuit element layer 13. The second method is to set the focusing position of a laser beam on the internal portion of the wafer 12 to form a melt processing region 16 due to multiphoton absorption as shown in FIG. 2 and then discretely divide the wafer 12 by stretching a dicing film 15.
Since the first method requires extremely large laser power when the thick wafer 12 is cut, larger damages will be applied to the wafer in comparison with a case of the blade dicing method in some cases. Therefore, the first method is suitable for a process of cutting only the circuit element layer 13 on the surface layer by use of relatively low laser energy, for example.
Since the second method is to divide the wafer 12 starting from the melt processing region 16, it can cope with the relatively thick wafer 12. However, since insulating films and wiring layers (or inter-connections) are arranged in a complex form in the wiring pattern of the circuit element layer 13 on the surface layer, there occurs a possibility that unexpected destruction such as separation of the insulating film and layer-layer separation of the wiring layers will occur at the time of dividing. Particularly, there occurs a strong possibility that insulating films with small dielectric constant which are recently actively used will be destroyed at the time of dividing because of characteristics such as the low mechanical strength and low adhesion strength thereof.
That is, in the laser dicing, the surface state of a finished to-be-cut member is largely influenced. More specifically, if focusing and power adjustment are made on a region in which metal wiring layers are present and the wafer is cut, large damage is given to a region in which no metal wiring layers are present and film separation occurs in the worst case. Particularly, when a plurality of transparent films which permit the laser beam to pass therethrough are laminated on a dicing line region, the damage becomes significant. On the other hand, if focusing and power adjustment are made on a region in which no metal wiring layers are present, there occurs a possibility that the metal wiring layer cannot be cut in some cases.
Therefore, when the laser dicing is performed, it is required to finely adjust a laser dicing device according to the surface state of a to-be-cut member. However, since patterns of metal wiring layers such as alignment marks and test pads are arranged in a complicated form in the dicing line region of the actual semiconductor wafer, it is difficult to adjust and set the laser dicing device into an optimum state.
As described above, the conventional semiconductor device and the manufacturing method thereof have a problem that the surface state of a finished to-be-cut member is influenced when the laser dicing is performed, the quality is lowered and the cutting property and manufacturing yield are lowered.